To increase fuel economy, fuel delivery to an engine is shut-off when no torque is requested from the driver or from another device demanding torque (e.g., cruise control or adaptive cruise control (“ACC”)) for a predetermined time. When the engine receives a fuel shut-off command (e.g., from an engine controller or a separate fuel controller or velocity controller), the engine enters a fuel shut-off state where no fuel is supplied to the engine. When engine torque is requested and the engine is in the fuel shut-off state, the engine exits the fuel shut-off state and enters a minimal injection state. In the minimal injection state, a minimal amount of fuel is injected to the engine for providing torque. From the minimal injection state, the engine increases the amount of injected fuel or switches to another injection state to obtain the necessary fuel for providing the requested torque.
The fuel shut-off state and the minimal injection state are each associated with a particular torque output of the engine. If torque is requested from the engine that is between these torque outputs, the engine cannot directly provide the requested torque. In addition, in these situations, the engine often oscillates between the two states while providing inadequate or overcompensated torque. Uncontrolled oscillation between the two engine states reduces fuel efficiency and mitigates or negates the benefits of fuel shut-off.
Switching the engine from the fuel shut-off state to the minimal injection state also takes time. For example, in some situations, it takes approximately 0.5 to 1.0 second to return the engine to a fuel state (e.g., the minimal injection state) after the engine has been placed in a fuel shut-off state. Also, during the time required to switch engine states, the engine cannot immediately respond to commands or requests. Therefore, the delay also creates a problem for systems and controllers communicating with the engine, such as an adaptive cruise control (“ACC”) system. In particular, the time between a controller issuing a torque request to the engine and receiving a response is not constant, but varies based on whether the engine is in a fuel shut-off state.
Furthermore, when the engine is in the fuel shut-off state, there is no way for a system or controller to receive a quick engine response because of the delay required for switching engine states. In addition, if the system or controller attempts to make a request for a quick engine response, the engine usually responds with overcompensated torque too late. In addition, if the vehicle continuously oscillates between the two states as described above and the vehicle experiences torque overshooting each time the engine exits the fuel shut-off state, the driver experiences uncomfortable vehicle control.
To overcome the above problems, some ACC systems are configured to disable fuel shut-off when the ACC is functioning. Other ACC systems only allow fuel shut-off when the ACC system is braking the vehicle. Still other ACC systems use a combination of the above states based on the vehicle's current speed or acceleration. These approaches, however, do not solve all of the problems with fuel shut-off and often reduce or eliminate the fuel economy benefits associated with fuel shut-off.